Update colab examples

[wenxindongwork]

Use transformers' AutoModelForCausalLM instead of optimum-tpu's AutoModelForCausalLM for finetuning.

The from optimum.tpu version imports models that are specifically optimized for inference. While the colab example works for smaller models, it fails with a HBM OOM error for llama3-70b (on a v4-256). Changing the following import statement solved the problem.

Before submitting

• [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [x] Did you make sure to update the documentation with your changes?
• [x] Did you write any new necessary tests?

[wenxindongwork]

Just updated the examples to load the models in bf16 instead, hope that works!

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[tengomucho]

The code style workflow is failing, can you run it locally (make style) and push again so we can merge this please?

[wenxindongwork]

just installed ruff and ran make sytle. Thanks!